Here we go again: Dominiques v. Barred Rocks

I received my six chicks from My Pet Chicken on May 22. I assume they were born around the 20th. From almost the very beginning I could divide them in half based on their appearance. As they have grown, three have lagged behind the others in their development. One of the more developed ones died on the Friday 13th incident, but Chicken Matthews and Chicken Jo are much more developed than Spunky, Chicken Georgette and Dolly. The main difference seems to be in tail feathers. The first two look like nice little hens with nice tail plumage. The others barely have tailfeathers and still look like chicks. And I've also noticed as their combs start to grow, Chicken Jo has a straight, single comb. Chicken Matthews' comb is not quite as single, but not nearly as rose shaped as the other three. Do BRs develop more quickly than Doms? Will they begin laying more quickly? I've also noticed this doesn't seem to affect the pecking order. Spunky, my smallest and least developed, really lives up to her name in that department and hardly ever backs down to her "colleagues."

Any comments from you breed experts? I wish I could post pix but we live in the dark ages and only have a traditonal film camera.

Thanks.

PS - when should I start giving them oyster shells? As they approach laying age or after they start laying?